1. Field of the Invention
This invention relates to a method for preparing 3,3-dimethylbutyraldehyde in a highly economical manner by regioselective isomerization of vaporized 1,2-epoxy-3,3-dimethylbutane in the presence of silica gel. The invention also relates to a method for preparing 1,2-epoxy-3,3-dimethylbutane from 3,3-dimethylbutene by oxidation with dimethyldioxirane.
2. Related Background Art
Rearrangement of epoxides to carbonyl compounds has long been known. See, e.g., Smith, J. G. Synthesis, 629 (1984). For example, epoxides have been converted to aldehydes in a regiospecific manner through the use of lithium perchlorate in dimethyl ether (LPDE). Sudha, R., et al. J. Org. Chem. 61, 1877 (1996). However, this reference discloses that acyclic terminal olefin epoxides, such as 1,2-epoxyhexane did not react in the LPDE medium.
Yet another example is provided by Lemini, C., et al., Synth. Commun. 25, 2695 (1995) which report the transformation of arylmonosubstituted and 2-aryl, 2-methyl disubstituted oxiranes to aldehydes using silica gel as a reagent in solution under very mild conditions. U.S. Pat. No. 2,660,609 is directed to the isomerization of alkylene oxides such as ethylene oxide, 1,2-propylene oxide or 1,2 butylene oxide to aldehydes in the vapor phase using silica gel or fuller's earth in a fluidized form. However, these references do not disclose or suggest the regiospecific isomerization of 3,3-dimethylbutyraldehyde with silica gel.
Moreover, the latter reference also describes the formation of ketones and alcohols along with aldehydes in the isomerization of 1,2-epoxyalkanes with silica. The rearrangement gives different products (aldehydes, ketones, or alcohols) depending upon the reagents, reaction conditions and substituents (reviews on the isomerization of epoxides to aldehydes: J. G., Synthesis 1984, 637; Rickborn, B. in Comprehensive Organic Synthesis, ed. B. M. Trost, I. Fleming and G. Pattenden, Pergamon, Oxford, 1991, vol. 3, p. 733-775). Therefore, the behavior of the tert-butyl group is not predictable from that of the straight or other branched chain alkanes. For example, rearrangement of 1,2-epoxyalkanes (House, H., J. Amer. Chem. Soc. 1955, 77, 5083; where R is alkyl group) with boron trifluoride etherate gives only aldehyde and no ketone, whereas when R is a tert-butyl group, a mixture of products or the starting material are recovered (unpublished results). With LiBr-basic alumina only ketone, pinacolone, is formed (unpublished results). The effect of alkyl groups in the rearrangement of 1,2-epoxyalkanes using LiBr-HMPA is also observed by Rickborn and Gerkin (Rickborn, B. and Gerkin, R. M., J. Am. Chem. Soc. 1971, 93, 1693). Therefore, the conditions effective for n-alkyl or certain branched chain alkyl groups would not necessarily be expected to be effective for the tert-butyl group.
3,3-Dimethylbutyraldehyde is an intermediate that is useful in the preparation of the sweetener N-[N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl]-L-phenylalanine disclosed in U.S. Pat. No. 5,480,668 and U.S. Pat. No. 5,510,508. Accordingly, a method for preparing that intermediate which is both economical and specific is highly desired.